Arrangement and method to apply diffusing wave spectroscopy to measure the properties of multi-phase systems, as well as the changes therein

ABSTRACT

Arrangement for measuring physico-chemical properties of liquid, such as solutions, dispersions and emulsions. The arrangement comprises a light source for producing and emitting light in the liquid, a detector for detecting said light after being scattered by said liquid, processing means arranged for receiving an output signal from said detector. The processing means is further arranged to calculate a maximum value of the mean square displacement &lt;Δr m   2 &gt; from the autocorrelation function g (2)  as a function of time and the value of the property from said calculated maximum value of the mean square displacement &lt;Δr m   2 &gt;.

BACKGROUND OF THE INVENTION

The present invention relates to an arrangement for measuring theproperties of multi-phase systems as well as the changes therein, suchas changing interactions between particles in a solution. In particular,the invention relates to such an arrangement for use with multi-phasesystems in which at least one of the phases is a liquid phase or fluid,such as gels, lattices, suspensions or emulsions.

The arrangement of the invention comprises a light source for producinga light beam. It is possible to use at least one source fibre having afirst end arranged for receiving said laser beam and a second endarranged for emitting light in a multi-phase system, or to point thelight directly into the multi-phase system. At least one detector fibre,arranged for detecting said light after being scattered by saidmulti-phase system, processing means arranged for receiving an outputsignal from said at least one detector fibre and for calculatingpredetermined parameters with respect to said multi-phase system.

Such an arrangement is known from D. S. Horne, Dynamic Light ScatteringStudies of Concentrated Casein Micelle Suspensions, Chapter 15 in S. E.Hardings, e.a. Laser Light Scattering in Biochemistry, 1992.

The Diffusing Wave Spectroscopy (DWS) arrangement described by Horne,comprises a bifurcated optical fibre bundle as light guide. Half thefibres are connected to a randomly polarized He—Ne laser. The other halfof the fibres is connected to a photomultiplier. The bundle of fibres isdistributed randomly over the face of a common leg. In use, thenon-connected ends of the fibres are dipped into a scatteringmulti-phase system, e.g. milk or a milk derived medium/solution. Thosefibres which are connected to the laser emit laser light into themulti-phase system. Light backscattered by the multi-phase system isdetected by the fibres connected to the photomultiplier. Masking by aslit and a pinhole may ensure that light from only a small area impingeson the detector.

With such an arrangement, intensity correlation functions can bemeasured. Examples of such functions are presented for 330 nmpolystyrene latex and undiluted skim milk. Moreover, Horne shows thatrelaxation time as a function of casein micelle volume fraction inreconstituted milk can be measured, Horne also shows that the relaxationtime changes due to curd formation. Thus, the transition from fluid togel can be detected. One of Horne's conclusions is that: “It thereforeappears that observation of DWS behaviour in these gelling systems, byvirtue of its measurement of relaxation in the system, must eventuallyreflect changes in visco-elastic properties”. However, Horne does notindicate how visco-elastic properties may be quantitatively derived fromDWS measurements.

Moreover, the measurements described by Horne can not easily be madequantitative because these measurements are made with a randomlydistributed bundle of incoming and outgoing optical fibres.

A. C. M. van Hooydonk, e.a., Control and Determination of theCurd-setting during Cheesemaking, Bulletin of the International DairyFederation (1988) (No. 255), pp. 2-10, observe that a lot of scientificresearch has been devoted to rennet-induced coagulation of milk.However, up to now no quantitative measurement of gel formation and thesubsequent synersis of the curd is available. The optimum coagulumfirmness must be determined on-line for cutting to obtain maximum cheeseyield and cheese quality. Hooydonk e.a. note: “Up to now mostcheesemakers judge the optimum cutting time by the “feel” of the curdand . . . with amazing accuracy”. Moreover, they note that although manyinstruments have been developed to carry out this task automatically,none of them have been widely accepted. The so-called “Gelograph” isconsidered as a standard instrument for measuring the gelation ofcheesemilk at low gel strength. However, due to ongoing automation andincrease of scale of cheesemaking plants there is a strong interest inautomatic methods for monitoring the process of curd-setting.

In the U.S. Pat. No. 4,975,237 a dynamic light scattering apparatus isdisclosed, comprising a laser as light source, optically coupled to alight scattering sample via a first optical fibre and a first lens. Thelens produces a beam waist in a sample and scattered light is collectedby a receive lens and a receive fibre. A photodetector detects lighttransmitted by the receive fibre and converts it in an electricalsignal. The photodetector is connected to a correlator and computer.This correlater is not used for quantitative measurements ofcharacteristics of the sample.

A primary object of the present invention is to provide an apparatuswith which the properties of liquids, such as solutions, dispersions andemulsions can be measured and to relate physico-chemical properties tolight scattering measurement in liquids.

SUMMARY OF THE INVENTION

For the purposes of the invention, the term “liquids” comprises bothheterogeneous systems which contain two distinct phases, such as aliquid phase and a suspended solid phase, two immiscible liquid phases,or an emulsified (liquid) phase in a liquid phase, as well as morehomogeneous systems which are subject to phase changes, phasetransitions or phase formation, such as systems in which gel formation,coagulation, aggregation or changes in viscosity can occur.

These homogenous or heterogenous systems can comprise organic, inorganicas well as biological media or components, aqueous systems or solutions,or systems of a mixed organic/inorganic and/or biological nature. In aparticular embodiment, the multi-phase system is milk or a milk derivedmedium/solution, for instance as used in cheese-making.

A further object of the present invention is to provide a method whichcan be carried out by an apparatus according to the invention and whichis able to provide physico-chemical properties of such liquids by meansof diffusing wave spectroscopy. Such a method may be related tomonitoring the renneting of cheesemilk during cheese-making, but is notrestricted thereto.

Thus, the arrangement according to a first aspect of the presentinvention as defined above is characterized in that a processing meansis arranged to calculate a maximum value of the mean square displacement<Δr_(m) ²> from the autocorrelation function g⁽²⁾ as a function of timeand the value of the physico-chemical property from said calculatedmaximum value of the mean square displacement <Δr_(m) ²>.

In an embodiment the physico-chemical property is the gel-strength G′which is calculated using the following equation:$G^{\prime} \approx \frac{k_{B}T}{\xi {\langle{\Delta \quad r\frac{2}{m}}\rangle}}$

in which,

k_(b)·T=thermal energy of particles in the gel;

ξ=size of a cluster in the gel.

The arrangement according to a second aspect of present invention ischaracterized in that a processing means is arranged to determine thehalf decay time as a function of time of the autocorrelation functionand to determine the value of the physico-chemical property using apredetermined relation between the half decay time and theautocorrelation function. This value may be the gelstrength.

A further difference between the arrangement according to the inventionand the Horne arrangement is the configuration of the optical fibres.Whereas the fibres in the Horne arrangement are distributed randomly andthe mutual distances between the fibres is unknown, in the arrangementaccording to the invention the mutual distances between the detectorfibres are predetermined. In order to facilitate the calculations thedetector fibres are, preferably, single-mode fibres suitable for onespecific monochromatic wavelength. The fibres are preferably set up inthe so-called “back-scattering geometry”, which makes it possible toquantify the autocorrelation function that is measured. They may havethe shape of a dipstick, so they can be easily stuck into any kind ofliquid.

The advantage of DWS in respect to the gelograph is that changinginteractions between the (droplets, bubbles or particles of the) phrasescan be measured. In the case of renneting of cheesemilk the effects ofthe addition of rennet can be observed in a much earlier stage of theprocess. The method is non-destructive, because its working principle isnot based on mechanical principles, but on the scattering ofmonochromatic light that does not damage the liquid.

Furthermore, the DWS can be used to do local measurements, which alsomakes it possible to probe inhomogeneities in samples. The volume thatis probed by one pair of source fibre and detector fibres ranges from 1nl to 1 l or more and can be located in an infinitely large volume. Itdepends on the distance between the source fibre and detector fibres.

In one embodiment of the arrangement according to the invention a sourcefibre and the at least one detector fibre are immersed in a milk with anaddition of a rennet.

In order to automatically cut cheesemilk at the proper gel-strength, ina further embodiment the arrangement comprises a cutting machine coupledto the processing means, wherein the processing means are arranged tocompare the gelstrength with a reference gelstrength, and to activatethe cutting machine upon the gelstrength reaching the referencegelstrenth for cutting the gel.

Moreover, a third aspect of the invention is directed to a method formeasuring the physico-chemical properties of liquids such as solutions,dispersions and emulsions, comprising the steps of:

producing a light beam (2)

emitting light in a liquid (7);

detecting said light after being scattered by said liquid (7);

converting the detected light in an electrical signal;

transmitting said electrical signal to processing means;

calculating the autocorrelation function g⁽²⁾ in the time- or thefrequency domain of said electrical signal characterized in that amaximum value of the mean square displacement <Δr_(m) ²> is calculatedfrom the autocorrelation function g⁽²⁾ (τ), or from g⁽²⁾ (ν) in asimilar way, as a function of time and the value of said property iscalculated from said calculated maximum value of the mean squaredisplacement <Δr_(m) ²>. The value may be the gelstrength which iscalculated using the following equation:$G^{\prime} \approx \frac{k_{B}T}{\xi {\langle{\Delta \quad r\frac{2}{m}}\rangle}}$

in which,

k_(b)·T=thermal energy of particles in the gel;

ξ=size of a cluster in the gel.

A fourth aspect of the invention is directed to a method for measuringphysico-chemical properties of liquids such as solutions, dispersionsand emulsions, comprising the steps of:

producing a light beam (2)

emitting light in a liquid (7);

detecting said light after being scattered by said liquid (7);

converting the detected light in an electrical signal;

transmitting said electrical signal to processing means;

calculating the autocorrelation function g⁽²⁾ of said electrical signal,characterized in that the half decay time is calculated as a function oftime of the autocorrelation function and the value of said property isdetermined using a predetermined relation between the half decay timeand the autocorrelation function. The value may be the gelstrength.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be explained in detail with reference to somedrawings which are intended to illustrate the present invention and notto limit its scope.

FIG. 1 shows a schematical block diagram of an arrangement according tothe invention;

FIG. 2 shows autocorrelation functions for different distances betweenthe source fibre and the detector fibres;

FIG. 3 shows the same data as FIG. 2 but normalized for the distancesbetween the source fibre and the detector fibres;

FIG. 4 shows intensity profiles for different types of milk, as afunction of source detector distance;

FIG. 5 shows autorcorrelation functions at different times after theaddition of rennet to milk for a source detector distance of 4.9 mm;

FIG. 6 shows the same data as FIG. 5 but for a source detector distanceof 9 mm;

FIG. 7 shows half decay times for different additions of rennet to skimmilk as a function of time after the addition of the rennet;

FIG. 8 shows intensity profiles for different additions of rennet toskim milk as a function of time after the addition of the rennet;

FIG. 9 compares the time dependency of the half decay time and theintensity profile for an addition of 0.02% rennet to skim milk;

FIG. 10 compares half decay times for skim milk with those for wholemilk as a function of time after that addition of rennet;

FIG. 11 compares the time dependency of the half decay time and theintensity profile for an addition of 0.02% rennent to whole milk.

DETAILED DESCRIPTION

FIG. 1 shows a possible embodiment of a diffusing wave spectroscopyarrangement according to the invention. The arrangement shown comprisesa laser 1, which may be an Ar⁺ laser with a wavelength of 514 nanometre.The laser 1 generates a laser beam 2 which is directed to a collimator 3which produces a collimated laser beam 4 directed to a multimode fibre5. The laser light may be coupled into the multimode fibre 5 by means ofa fibre coupler F220SMA-S, Thorlabs, mounted on a tilting stage. Themultimode fibre 5 may be a fibre FG-50-GLA, Thorlabs, having a corediameter of 50 micrometer. In use, the multimode fibre 5 is immersedwith one end in a multi phase system/solution 7, e.g. milk, in acontainer 6.

In the arrangement shown in FIG. 1, there are shown five detector fibres8(1), . . . , 8(5), located at predetermined distances from themultimode fibre 5. However, the number of detector fibres may be 1 ormore. The detector fibres are preferably single mode fibres. They areconnected to a fibre multiplexer 9 which may be a Dicon fibremultiplexer.

The output of the fibre multiplexer 9 is connected via a connection 10to a photomultiplier tube 11, e.g. a HC-120PMT, Hamamatsu, which is inturn connected to an amplifier/discriminator 12. Theamplifier/discriminator 12 is connected to a processor 13 whichcomprises a computer mounted correlator board, e.g., a BrookhavenInstruments 9000. Data acquisition and analysis will be carried out byprocessor 13. The processor 13 is connected to a memory 15 and inputmeans like a keyboard, a mouse, etc.

In one embodiment the processor 13 is connected to a cutting machine 14for cutting gelated milk when it has reached the proper gelstrength in acheese making process.

In use, the multimode fibre 5 emits laser light into the multi-phasesystem 7. The laser light is multiple scattered by the multi-phasesystem and the multiple scattered laser light will be detected by thedetector fibres 8 (1), . . . , 8 (5). These detector fibres may besingle mode fibres FS-VS-2614, Thorlabs. With the fibre multiplexer 9one channel picking up the signal of one of the detector fibres 8 (1), .. . , can be selected for measurement of a correlation function.

In one experiment the multi-phase system 7 was fresh skim milk. Whereasin another experiment the multi-phase system 7 was whole milk. Both wereobtained from Wawa supermarkets.

Skim milk did contain a negligible amount of fat, whole milk contained33 g/l fat. The milk was poured in container 6 (6.6 cm×12.0 cm×11.0 cm)and brought to a temperature of 30° C. by putting the container in awater batch (not shown). The temperature of the milk was controlled byreplacing cold water with hot water at regular intervals. In this way,the temperature could be kept within 2° C. from 30° C.

Rennet was added at a usual concentration of 20 ml per 100 1 (0.02%) ofmilk. To skim milk higher concentrations of 0.04% and 0.08% were alsoadded. In the first experiments autocorrelation functions andintensities of detected, backscattered laser light were measured foreach channel during 60 s at relatively large intervals. Later, thecorrelation functions and the intensities on the channel, which has asource detector separation of 4.9 mm between the end of multimode fibre5 and the end of detector fibre 8 (1) immersed in multi-phase system 7,was measured in 30 s in 60 s intervals during a period of several hours.The fibre multiplexer 9 was left in the setup, but did not have afunction in these latter measurements.

Before showing some measurement results, first, some theory will begiven.

The expression for the intensity in an indefinite system is:$\begin{matrix}{{I(\rho)} = {\frac{A\quad \mu_{s}^{\prime}}{\rho}{\exp \left( {{- \sqrt{3\quad \mu_{a}\mu_{s}^{\prime}}}p} \right)}}} & (1)\end{matrix}$

wherein A is a constant, depending on e.g. the laser input power and thecoupling efficiencies of the fibres, μ_(a) is the absorption coefficientand ρ is the source-detector separation. μ_(e)′ is the reciprocal of themean free path, 1* . The electric field correlation functiong⁽¹⁾(τ)(=<E_(s)*(0)E_(s)(τ)>/<E_(s) ²>) for the backscattering geometryis given by: $\begin{matrix}{{g^{(1)}(\tau)} = {\exp \left\{ {{- \rho}\quad \mu_{s}^{\prime}\sqrt{{{\langle{\Delta \quad {r^{2}(\tau)}}\rangle}q\frac{2}{0}} + {3\frac{\mu_{a}}{\mu_{s}^{\prime}}}}} \right\}}} & (2)\end{matrix}$

wherein Δr²(τ) is the mean square displacement; q₀ is the wave vectorrelated to the wave length and τ is the measuring time between twosuccessive measuring points.

Since the autocorrelation function g⁽²⁾ (τ) of the intensity I(g⁽²⁾(τ)=<I(o)·I(τ)>/<I²(o)>) is given by the Siegert relation.

 g⁽²⁾(τ)=1+β|g ⁽¹⁾(τ)|²  (3)

herein β is a constant related to the arrangement used, theautocorrelation function g⁽²⁾(τ) can be calculated. In the experimentsthe baseline at large τ, where g⁽¹⁾(τ)=0, is measured and used tocalculate g⁽²⁾(τ). The factor β, which should be equal to 0.5 because asingle mode fibre 8(1), . . . is used, was estimated from the behaviourof g⁽²⁾(τ) for short correlation times. Finally, the base line wassubtracted from the data and the data were normalized for β, giving(g⁽²⁾(τ)−1)/β, which is equal to |g⁽¹⁾(τ)|².

For Brownian motion of particles, the mean square displacement can bewritten as <Δr²>=6 Dτ, with D the diffusion constant. In this case ln(g⁽¹⁾(τ))/ρα√τ, and the experimental data should form one common line ina graphical representation.

FIGS. 2 and 3, respectively, show ln(g⁽²⁾(τ)) and ln(g⁽²⁾(τ))/ρ,respectively, as a function of √τ as measured by five fibres (8(1), . .. 8(5) in skim milk, located at different distances from the sourcefibre 5 as specified in the righthand portions of FIGS. 2 and 3.

The laser output used for the first experiments was about 300 mW.ln(g⁽²⁾(τ)) vs √τ will give a straight line for the Brownian behaviour.The datapoints in the ln(g⁽²⁾(τ))/ρ vs √τ curves are all on the sameline, as predicted by the theory (FIG. 3), until g⁽²⁾(τ) reaches thenoise level. This is most obvious for the channel with the largestsource-detector separation ρ=1.92 cm.

The intensity dependence on the source-detector separation ρ is given inFIG. 4. It was assumed that the fibre/multiplexer couplings all hadapproximately the same efficiency. The lines are a best fit, with a andμ_(a)′ as variables. μ_(a) is fixed at a value of 0.03 cm⁻¹. For A andμ_(a)′, respectively, we find 31 and 25.7 cm⁻¹, respectively, in skimmilk and 21 and 67.7 cm⁻¹, respectively, in whole milk. This means thatthe mean free path 1* in skim milk is 0.39 mm and 0.15 mm in whole milk.This is caused by scattering of the fat globules in whole milk.

The time-dependence of the correlation functions was followed after theaddition of rennet (0.02%) at time 0. The correlation functions wererecorded at t<0 (i.e. before addition of rennet), t=10 min. t=40 min.and t=120 min. FIGS. 5 and 6 give the time-dependence of the correlationfunctions for skim milk measured on the channel where ρ=4.9 mm and whereρ=9.1 mm, respectively. These figures show typical correlation functionsln(g⁽²⁾(τ), t) for milk at t=0 and t=10 min, during the gel formation att=40 min, and after the gel has formed at t=120 min.

The correlation functions were also measured during 30 s at 1 minintervals to gain more insight in their development in time. Since thechange of the correlation functions during gelling seemed to proceedsimilarly for all source-detector distances between 5 and 19 mm, onlythe correlation functions for the nearest channel (ρ=4.9 mm) weremeasured. The power output of the laser 1 was turned down to about 30mW. The resulting count rate was in the order of 100 kHz. The change inthe correlation functions g⁽²⁾(τ) can be expressed in terms of the halfdecay time, i.e. the time it takes the correlation function g⁽²⁾(τ) todrop to half of its original value. These measurements were performedfor skim milk with rennet concentrations of 0.02%, 0.04%, and 0.08%, andfor whole milk with a rennet concentration of 0.02%.

The change in the half decay times is shown in FIG. 7 in a log-lineargraph. When the rennet concentration is increased the gel formationstarts sooner, as expected. The half decay time increases fromapproximately 2.5 μs for milk to 100 μs for the gel. Before the gelformation starts, i.e. in the period that aggregates are formed, thehalf decay time decreases slightly. In this same period the intensityprofile has a maximum as shown in FIG. 8, that shows the intensity as afunction of time in skim milk with different rennet concentrations.

For a better comparison both the half decay time and the intensity inskim milk with 0.02% rennet added are given in FIG. 9. The gel formationwas checked visually and started approximately at t=40 min when 0.02%rennet was added.

The same measurements were performed for whole milk, which has about3.5% fat. The fat is dispersed in fat globules that contribute to thescattering of the milk. This was shown in FIG. 4. The increase in thehalf decay time proceeds similarly to that in skim milk (FIG. 10).Visually, it was observed that whole milk also starts to form a gel att=40 min.

The half decay times in whole milk are smaller than those in skim milk.The half decay time increases from 1.5 μs to 25 μs. The ratio (halfdecay time for skim milk)/(half decay time for whole milk) increasesfrom approximately 2 for the milks to more than 4 for the gels.

FIG. 11 shows both the half decay time and intensity in whole milk afteran addition of 0.02% rennet. Comparing the profiles of the half decaytime and the intensity shows similar behaviour to that for skim milk.

From these measurements it can be concluded that the growth of thecaseine network forming the gel is not sensitive to the addition of thisamount of fat. This is very convenient for the application of DWS incheese making.

Moreover, viscoelastic properties like gel-strength can be directlyderived from the measurements made in the following way.

Gel-strength, which is given by the so-called Gelmodulus G′, can berelated to the mean square displacement. <Δr²> by making the followingderivation. The principle is that a moving particle is trapped by theelasticity of the gel, which leads to a balance between the thermalenergy of the particle and the strength of the gel: $\begin{matrix}{G^{\prime} \approx \frac{k_{B}T}{\xi {\langle{\Delta \quad r\frac{2}{m}}\rangle}}} & (4)\end{matrix}$

In this equation k_(b)T is the thermal energy of the particle, ξ is thesize of a cluster in the gel and <Δr_(m) ²> is the maximum value of themean square displacement. <Δr_(m) ²> can be derived in the followingway. Δr²(τ) is related to the electric field correlation functiong⁽¹⁾(τ) by equation 2. However, g⁽¹⁾(τ) is also a function of the time telapsed since the addition of rennet to milk, as is for instance evidentfrom FIGS. 5 and 6. Thus, Δr²(τ) is also a function of time t. It turnsout that Δr²(τ) has a maximum <Δr_(m) ²> for a certain τ which dependson the time t. <Δr_(m) ²> is a function of time t which can beautomatically monitored by means of equation 2.

The size of a gel-cluster is given by: $\begin{matrix}{\xi = {N\frac{1/d_{f}}{cp}a}} & (5)\end{matrix}$

where N_(cp) is the number of particles in the cluster, a is the size ofparticles (caseine micelles in cheese) and d_(f) is the fractaldimension of the cluster, which is approximately 2 in a cheese-gel. Thenumber of particles is also given by: $\begin{matrix}{N_{cp} = \frac{N_{p}}{N_{c}}} & (6)\end{matrix}$

with N_(p) the number of particles, originally present in the milk, andN_(c) the number of clusters in the gel. The volume fractions giveanother relation between a and ξ. $\begin{matrix}{\varphi_{0} = \frac{N_{p}V_{p}}{V_{t}}} & (7)\end{matrix}$

with φ₀ the initial volume fraction of casein micelles in the milk(=0.1). V_(p) the volume of a casein micelle (=4/3 Πa³) and V_(t) thetotal volume of milk. The same equation can be given for the gel, whichfills the whole volume, so φ˜1. $\begin{matrix}{1 = \frac{N_{c}V_{c}}{V_{t}}} & (8)\end{matrix}$

with V_(c) the volume of a cluster with radius ξ(V_(c)=4/3Πξ³).

Combining equations 6, 7, and 8 gives: $\begin{matrix}{N_{cp} = \frac{\varphi_{0}\xi^{3}}{a^{3}}} & (9)\end{matrix}$

Assuming df=2 and using equation 5, this eventually leads to thefollowing simple expression for ξ: $\begin{matrix}{\xi = \frac{a}{\varphi_{0}}} & (10)\end{matrix}$

From equations 4 and 10 the following equation can be derived:$\begin{matrix}{G^{\prime} = \frac{{\varphi_{0} \cdot k_{B}}T}{a{\langle{\Delta \quad r\frac{2}{t}}\rangle}}} & (11)\end{matrix}$

In this equation a, K_(B)·T and φ₀ to are constants and <Δr_(m) ²> canbe monitored as function of the time t. Thus, G′ can be monitored as afunction of the time t by processor 13. By comparing G′(t) with areference value G′_(ref) stored in memory 15 and indicating the idealgel-strength for cutting, the cutting by cutting machine 14 can beautomatically started as soon as G′(t) equals G′_(ref). The memory 15can store a table with different values for the G′_(ref) for differenttypes of milk, with different types and/or amounts of added rennet. Theoperator can then select the proper value of G′_(ref) by means of inputmeans 16.

Alternatively, the electric field correlation g⁽¹⁾(τ) can also berelated to the half decay time τ_(1/2)(t). Then, by means of equation 11the gel-strength G′(t) can be expressed as a function of the half decaytime τ_(1/2)(t). Thus, the processor 13 can calculate the gel-strengthG′(t) by measuring the half decay time τ_(1/2)(t). The advantage of thisalternative method is that the half decay time τ_(1/2)(t) appears toshow a rapid change of value in the area where gelation occurs and whereG′(t) reaches the reference value G′_(ref). This latter feature resultsin a possibly very accurate measurement of the moment at which gelationoccurs.

Furthermore, the method could even give at least an indication about thefat content of the milk that is being used by measuring the half decaytime at known added percentage of rennet (of FIG. 10).

These experiments show also that to monitor the gel formation withoutthe need to calculate the adsorption and scattering coefficients oneoutput fibre could be used instead of 5, provided that a suitablesource-detector distance is applied. This would allow the elimination ofthe fibre multiplexer from the experimental setup.

The laser power could be turned down to about 30 mW when thesource-detector distance was 4.9 mm. Smaller distances could be used,enabling the use of lasers with even lower powers, until the distancebecomes too small to ensure only multiple scattering. The DWS theorywill no longer hold if the single scattering regime is reached. Thiswill be the case if the source detector distance is in the order of themean free path, which is about 0.4 mm in skim milk.

Above, the method according to the invention has been explained withreference to measuring/monitoring renneting of milk.

However, it can be applied in a much broader field. In particular, theinvention can be applied to any system, process, medium, reaction,mixture, suspension, emulsion, etc. in which at least two phases occursimultaneously —i.e. in the same holder, vessel or reactor—and in whichat least one of the phases is a liquid (phase), a fluid phase, and/or asolution.

The further phase(s) present besides said liquid phase is notparticularly limited as long as it can be distinguished from said firstliquid phase for the purposes of the invention, i.e. in that itinfluences the scattering of a laser beam that is emitted therein ortherethrough.

As such, said further phase(s) can for instance be a solid phase, suchas a flocculated solid phase, a suspended solid phase and/or a suspendedparticulate material; a semi-solid or liquid phase, which for thepresent purposes includes gels, aggregated states, suspended droplets ofan immiscible liquid, emulsified droplets including those which occur inknown W/O-emulsions, O/W-emulsions and multiple and/or invertedemulsions such as O/W/O-emulsions and W/O/W emulsions, as well asmicelles; or even a gaseous phase, such as small bubbles of a suspendedgas or bubbles of a gas which is led through the liquid phase. In thisrespect, it should be noted that in the general disclosure above, theterm “particles” is used to denote any such further phase present, whichis preferably distinct from the liquid phase and of small size.

The invention can be used to measure the above indicated properties ofsuch heterogeneous systems, and in particular changes in time thereof ascan occur spontaneously or —usually—as a result of a chemical reaction,a physical process or a biological process, and which lead to changes inlaser beam scattering.

For instance, the system can be used to measure the stability orhomogeneity of suspensions, emulsions or lattices. It can also be usedto follow the course of a chemical reaction such as a polymerization, aphysical process such as mixing or phase separation, or a biologicalconversation such as a fermentation, i.e. by following the changes inthe phase(s) and/or in the interactions between them over time; or byfollowing the changes in the composition or properties of one or more ofthe phases present.

Preferably, these are processes in which gel formation, coagulation,flocculation, aggregation or changes in viscosity play an importantrole. Also changing interactions, not necessarily leading to gelationcan be measured, for example when a so-called stabilizer is added to asuspension. It might even be used to probe particles in a clear gel ofe.g. polymers (Xanthan gels). Fields in which it can be applied include:fermentation, dairy (cheese, yoghurt, cream), fungi, paints, plasticsand polymers, cosmetics, the oil industry, and the chemical industry.

As will be clear, the invention can also be used to study phase changesor transitions of a more homogeneous nature, or of more homogeneous(i.e. essentially single phase) systems, such as can occur in gelation,coagulation, flocculation or aggregation, or even as a result of changesin temperature. As mentioned above, for the purposes of the invention,such systems are encompassed within the term “multi-phase system”, asthere are differences between the initial physical state (phase) and thefinal physical state (phase).

The invention can also be used to measure viscosity, homogenity etc. ofsuch systems, or changes therein over time.

The distance between the fibres falls typically in the range from 0.1 mmto 100 mm and can be adjusted. The particles are preferably in thecolloidal size range, which means that their maximum size is about 10μm.

The temperature boundaries are determined by what the optical fibres andthe collimators can endure. A rough estimate is −20° C.<T<80° C.,2<pH<9. About 1 measurement can be done in 5 sec with a relatively lowlaser power of 25 mW. When a laser with a higher power is used (e.g. 500mW) the measuring time can be reduced to 1 sec. Changes that are slowerthan the measuring time can be monitored with DWS. liquid, such assolutions, dispersions and emulsions. The

What is claimed is:
 1. Arrangement for measuring physico-chemicalproperties of liquid, such as solutions, dispersions and emulsions,comprising a light source (1) for producing and emitting light in theliquid (7), a detector for detecting said light after being scattered bysaid liquid (7), processing means arranged for receiving an outputsignal from said detector and for calculating the autocorrelationfunction g⁽²⁾ of said output signal from said detector characterized inthat the processing means is arranged to calculate a maximum value ofthe mean square displacement <Δr_(m) ²> from the autocorrelationfunction g⁽³⁾ as a function of time and the value of thephysico-chemical property from said calculated maximum value of the meansquare displacement <Δr_(m) ²>.
 2. Arrangement according to claim 1,characterized in that the physico-chemical property is the gelstrengthG′, characterized in the processing means is arranged to calculate thegelstrength G′ from said calculated maximum value of the mean squaredisplacement <Δr_(m) ²> using the following equation$G^{\prime} \approx \frac{k_{B}T}{\xi {\langle{\Delta \quad r\frac{2}{m}}\rangle}}$

in which k_(b)·T=thermal energy of particles in the gel: ξ=size of acluster in the gel.
 3. Arrangement according to claim 1, comprising amachine (14) coupled to said processing means and wherein the processingmeans are arranged to compare said value of the physico-chemicalproperty with a reference value of said property, and to activate themachine (14) upon the value reaching said reference value. 4.Arrangement according to claim 1, comprising a machine (14) for treatinga gel, which is coupled to said processing means and wherein theprocessing means are arranged to compare said gelstrength G′ with areference gelstrength (G_(ref)), and to activate the machine (14) uponthe gelstrength G′ reaching said reference gelstrength for treating saidgel.
 5. Arrangement according to claim 1, comprising at least onedetector fibre (8(1), 8(2), . . . ) arranged for detecting said lightafter being scattered by said liquid (7), processing means arranged forreceiving an output signal from said at least one detector fibre and forcalculating parameters with respect to said liquid (7) wherein any oneof the at least one detector fibre (8(1), 8(2) . . . ) are located atpredetermined locations with respect to each other.
 6. Arrangementaccording to claim 5, wherein any one of the at least one detector fibre(8(1), 8(2) , . . . ) are single mode fibres.
 7. Arrangement accordingto claim 5 wherein the source fibre (5) is a multimode fibre. 8.Arrangement according to claim 5 wherein the distances between any oneof the at least one detector fibre (8(1), 8(2), . . . ) and the point ofimpinging of light on the liquid are larger than the mean free path ofthe light beam in the liquid (7).
 9. Arrangement according to claim 5wherein the light is directed to and the at least one detector fibre isimmersed in a milk medium with an addition of a rennet.
 10. Methodaccording to claim 1 wherein the liquid is a milk medium with anaddition of a rennet.
 11. Arrangement for measuring physico-chemicalproperties of liquids, such as solutions, dispersions and emulsions,comprising a light source (1) for producing and emitting light in theliquid (7), a detector for detecting said light after being scattered bysaid liquid (7), processing means arranged for receiving an outputsignal from said detector and for calculating the autocorrelationfunction g⁽²⁾ of said output signal from said detector, characterized inthat the processing means is arranged to determine the half decay timeas a function of time of the autocorrelation function and to determinethe value of the physico-chemical property using a predeterminedrelation between the half decay time and the autocorrelation function.12. Arrangement according to claim 1, characterized in that thephysico-chemical property is the gelstrength.
 13. Method for measuringphysico-chemical properties of liquids such as solutions, dispersionsand emulsions, comprising the steps of: producing a light beam (2)emitting light in a liquid (7); detecting said light after beingscattered by said liquid (7); converting the detected light in anelectrical signal; transmitting said electrical signal to processingmeans; calculating the autocorrelation function g⁽²⁾ in the time- or thefrequency domain of said electrical signal characterized in that saidmaximum value or the mean square displacement <Δr_(m) ²> is calculatedfrom the autocorrelation function g⁽²⁾τ(r), or from g⁽²⁾(τ) in a similarway, as a function of time and the value of the physico-chemicalproperty is calculated from said calculated maximum value of the meansquare displacement <Δr_(m) ²>.
 14. Method according to claim 13,wherein the physico-chemical property is the gelstrength G′,characterized in that the gel-strength G′ is calculated from saidcalculated maximum value of the mean square displacement <Δr_(m) ²>using the following equation:$G^{\prime} \approx \frac{k_{B}T}{\xi {\langle{\Delta \quad r\frac{2}{m}}\rangle}}$

in which, k_(b)·T=thermal energy of particles in the gel; ξ=size of acluster in the gel.
 15. Method according to claim 13, wherein the valueof the physico-chemical property is compared with a reference value forcontrolling a machine for treating the liquid.
 16. Method according toclaim 13 wherein either a gelation, a flocculation or an aggregationprocess in the multi-phase system is monitored.
 17. Method according toclaim 13, comprising the steps of: detecting said light after beingscattered by said liquid (7) by means of at least one detector fibre(8(1), 8(2), . . . ); transmitting an output signal from said at leastone detector fibre to processing means; calculating parameters withrespect to said liquid (7), wherein any one of the at least one detectorfibre (8(1), 8(2), . . . ) are located at predetermined locations withrespect to each other.
 18. Method according to claim 17, wherein thedistances between any one of the at least one detector fibre (8(1),8(2), . . . ) and the point of impinging of light on the liquid arelarger than the mean free path of the laser light in the liquid (7). 19.Method for measuring physico-chemical properties of liquids therein,such as solutions, dispersions and emulsions, comprising the steps of:producing a light beam (2) emitting light in a liquid (7); detectingsaid light after being scattered by said liquid (7); converting thedetected light in an electrical signal; transmitting said electricalsignal to processing means; calculating the autocorrelation functiong⁽²⁾ of said electrical signal, characterized in that the half decaytime is calculated as a function of time or the autocorrelationfunction, and the value of the physico-chemical property is determinedusing a predetermined relation between the half decay time and theautocorrelation function.
 20. Method according to claim 19, wherein thephysico-chemical property is the gelstrength.
 21. Method according toclaim 20 wherein said gelstrength G′ compared with a referencegelstrength (G_(ref)), and a machine (14) is activated upon thegelstrength G′ reaching said reference gelstrength for treating saidgel.